The present invention relates to a method for producing oxide dispersion strengthened ferritic steel tube having an excellent neutron irradiation resistance and an excellent high temperature strength (creep rupture strength against internal pressure and the like), such as a fuel cladding tube, which is a core member of a fast reactor.
As a material having excellent neutron irradiation resistance and high temperature strength properties, there has been developed an oxide dispersion strengthened ferritic steel in which fine oxide particles were dispersed in a ferritic steel and various investigations and studies have been made to the tube fabrication work for fuel cladding tubes using the ferritic steel.
Since a fuel cladding tube required for severe dimensional accuracy has a small diameter and thin wall thickness, a method for producing the tube adopts tube fabrication work by cold rolling having high working degree.
However, an oxide dispersion strengthened ferritic steel cladding tube produced by cold rolling has a capillary crystal grain structure (fibrous structure) in which crystal grains were thinly lengthened in a direction of rolling. Thus, there was a serious problem that ductility and creep rupture strength against internal pressure in a circumferential direction of a tube (that is, a direction perpendicular to a direction of rolling), which is significant as a component of a fast reactor are low. Further, there was a problem that the oxide dispersion strengthened ferritic steel is hardened by repeating cold rolling, resulting in substantial difficulty in the cold rolling and also occurrence of cracks.
To improve these problems, an attempt has been made to sufficiently perform the heat treatment after cold rolling to coarsen the crystal grains and generate a recrystallization structure in which the crystal grains are grown in the circumferential direction of the tube. For example, Japanese Patent Laid-open Specification No. 8-225891/1996 discloses compositions which can generate a recrystallization structure by specifying the content of Y2O3 in an oxide dispersion strengthened ferritic steel and the amount of excessive oxygen.
Further to prevent the hardening of the steel by repeating cold rolling, Japanese Patent Application No. 2001-062913, filed Mar. 7, 2001, for example, suggests a method for producing an oxide dispersion strengthened ferritic steel tube, in which a tube of the desired shape is produced by repeating cold rolling and heat treatment three times or more. In this method, an intermediate heat treatment during the cold rolling is performed at a temperature lower than 1100° C. to recover and soften the strain and dislocation generated by working without generating a recrystallization structure, so that a cold rolling in the next step can be efficiently performed, and the final heat treatment is performed at 1100° C. or higher to generate a recrystallization structure.
However, even if the compositions of the above-mentioned oxide dispersion strengthened ferritic steel is adopted and the intermediate heat treatment during the cold rolling is performed, there still were the following problems.
That is, when the intermediate heat treatment is performed without generating a recrystallization structure, there was a necessity to perform the real and substantial intermediate heat treatment after cutting a specimen out of an end of a tube after each cold rolling and checking the presence or absence of a recrystallization structure and the softening degree in the specimen by a previous test to set the most suitable condition of the intermediate heat treatment.
Further, when the intermediate heat treatment was performed at lower than 1100° C., the tube is not sufficiently softened but only to a limited hardness of about 400 Hv. Thus, although a cold rolling in the next step is possible, cracking can generate and it has been impossible to perform a stable tube manufacturing working.